1. Field of the Invention
This invention relates to a bone replacement material.
2. Description of the Prior Art
Collapse of a vertebral body as a result of trauma or osteoporosis is referred to as a vertebral body compression fracture. In the method that is known for the treatment of this kind of fracture, the collapsed vertebral body is repaired by filling the inside thereof with a filler (such as a bone replacement material) through a vertebral arch using a transpedicular technique.
In this treatment method, a collapsed vertebral body is first returned to a substantially original shape, that is, a collapsed vertebral body is reduced, whereby a cavity is created therein. A filler such as a bone replacement material is then inserted into the cavity to repair the vertebral body.
Conventionally, in such a treatment method, the inside of the collapsed vertebral body is filled with a granular bone replacement material using a cylindrical member having a hollow passage extending from the proximal end to the distal end thereof.
However, when such a conventional bone replacement material is used, the hollow passage of the cylindrical member is likely to be clogged with the bone replacement material, so that there is a case that it is difficult to smoothly perform such a filling operation. Such clogging occurs due to the following reasons. Namely, since such a granular bone replacement material is formed by grinding blocks of the bone replacement material, shapes of the particles are not uniform, so that such a granular bone replacement material has poor fluidity. Further, the particles of the granular bone replacement material include relatively small particles of which diameter is less than 1 mm. Such small particles are likely to enter the space between the inner surface of the hollow passage and the outer surface of an ejector bar which is inserted into the hollow passage. Such small particles entering into the space is liable to give an adverse effect to the slidability of the ejector bar with respect to the hollow passage. For these reasons, when such a conventional bone replacement material is used, the hollow passage of the cylindrical member is likely to be clogged with the bone replacement material.
Further, when the conventional bone replacement material is used, the bone replacement material introduced into the cavity of the vertebral body is likely to remain near an opening of the distal end of the cylindrical member, thus it is difficult to pack a sufficient amount of the bone replacement material into the cavity. Furthermore, even when the cavity is filled with a large amount of the bone replacement material, it is difficult to increase the filling factor sufficiently due to the presence of many gaps among the granules. Moreover, the granular bone replacement material is not always to have a spherical shape, namely, some of the granules are formed with protrusions on their surfaces. Such granules are likely to be broken when a load is applied thereto, so that the volume of the vertebral body is diminished. As a result, it is difficult to keep the result of the operation over a long period of time.
Further, in addition to the above method, there is another method which uses a bone replacement material (bone cement) that has fluidity when it is introduced into a vertebral body but will be hardened in the vertebral body. However, when such a material is used, there is a risk that the material will flow out from the vertebral body to damage adjacent nerves, thus problems still remain in its safeness.